Since organic field effect transistors can be produced by a lower temperature process than inorganic semiconductors, a plastic substrate and a film can be used as a substrate and an element that is lightweight and does not break easily can be produced. Moreover, in some cases, an element can be produced by film formation using a method of applying or printing a solution containing an organic material. Furthermore, since there are a wide variety of materials which can be used for the investigation of transistors, material properties can easily be changed fundamentally if materials differing in molecular structure are used. Therefore, it is also possible to realize various, flexible functions, elements, and so on which are impossible with inorganic semiconductors by combining materials differing in function.
In field effect transistors, the voltage applied to a gate electrode acts on a semiconductor layer through a gate insulating layer, thereby controlling on and off of a drain current. Therefore, gate insulating layer materials to be used for organic field effect transistors are required to have properties superior in insulating property and electrical breakdown strength when having been formed into a thin film. Particularly in field effect transistors of a bottom gate type, a semiconductor layer is formed on a gate insulating layer. Therefore, the gate insulating layer material is required also to have such properties as affinity with an organic semiconductor for forming a good interface with the organic semiconductor and a flatness of a film surface that forms an interface with a semiconductor.
Although investigations have been done for various materials with respect to gate insulating layer materials to be used for organic field effect transistors, technologies of utilizing organic materials which need neither high temperature conditions nor complicated equipment for layer formation have recently attracted attentions.
Patent literature 1 discloses the use of an epoxy resin and a silane coupling agent in combination as a gate insulating layer material in an organic thin film transistor. Since problems with the stability of transistor performance are caused if the hygroscopicity of a gate insulating layer material is high, a hydroxyl group formed in a curing reaction of an epoxy resin is made to react with a silane coupling agent in order to solve this problem.
Non-patent literature 1 discloses the use of a resin prepared by thermally cross-linking polyvinylphenol and a melamine compound for a gate insulating layer. By cross-linking with the melamine compound, the hydroxyl groups contained in the polyvinylphenol are removed and the film strength is increased simultaneously. A pentacene TFT having this gate insulating layer has a small hysteresis and exhibits durability to a gate bias stress.
Non-patent literature 2 discloses the use of polyvinylphenol and a copolymer prepared by copolymerizing vinylphenol with methyl methacrylate for a gate insulating layer. The polarity of the whole film is reduced by making the hydroxyl group of vinylphenol interact with the carbonyl group of methyl methacrylate. A pentacene TFT having this gate insulating layer has a small hysteresis and exhibits stable electric properties.